*{
    margin:0;
    padding: 0;
    text-decoration: none;
    font-size: 14px;
}

#box{
    width: 100%;
    height: 500rem;
    background: linear-gradient(to bottom,lightblue,lightyellow,lightgreen);
}

#totop{
    position: fixed;
    right: 200px;
    bottom: 100px;
    width: 50px;
    height: 50px;
    border-radius: 50%;
    background:red;
    text-align: center;
    line-height: 50px;
    text-decoration: none;
    opacity: 0.5;
    filter: opacity(500);
    &:hover{
        opacity: 1;
        filter: opacity(1000);
    }
}